US 6887419 B2
A multi-material runner gate system allows effective de-gating of elastomeric materials that would otherwise stretch and/or flex reducing de-gating forces at the gate upon separation of the molds.
1. A method of de-gating a molded part formed by injection of an elastomeric material in a mold cavity, the elastomeric material being introduced into the mold cavity through a runner passage and the de-gating removing a runner formed in the runner passage from the molded part, the method comprising the steps of:
(1) molding a runner support structure within a portion of the runner passage by the injection of a substantially inelastic material into the runner passage, the substantially inelastic material stopping before entry into the mold cavity from the runner passage, the inelastic material being substantially less elastic than the elastomeric material;
(2) molding the part by the injection of elastomeric material into the mold cavity through the runner passage past the substantially inelastic material into the mold cavity adjacent to the portion; and
(3) de-gating a runner composed of both the substantially inelastic material and the elastic material by removing the runner from the molded part, the substantially inelastic material in the runner reducing stretching and breaking of the elastic material of the runner at locations away from the junction of the runner and the molded part.
2. The method of
3. The method of
(i) fitting a blocking structure into a portion of the runner passage to prevent complete filling of the runner passage with the substantially inelastic material during the molding of the runner support structure, and
(ii) removing the blocking structure prior to step (2).
4. The method of
(i) fitting a blocking structure into a portion of the runner passage to prevent complete filling of the runner passage with the elastomeric material during the molding of the molded part, and
(ii) removing the blocking structure prior to step (1).
5. The method of
6. The method of
7. The method of
8. The method of
This invention relates generally to injection molding and more particularly to a mold that provides improved de-gating of parts molded from elastic material.
In injection molding, a thermoplastic material is forced under pressure into a mold cavity that defines the shape of the part being molded. A nozzle, providing the source of plastic material, communicates with the mold cavity through a sprue passage joining with one or more runner passages, in turn connecting to the mold cavity at corresponding gates. Generally, the gates are somewhat narrower than the runner passages to allow the sprue and runners (referring respectively to plastic remaining in the sprue and runner passages at the completion of the molding) to be broken away from the molded part at the gate in a process termed “de-gating”.
De-gating may occur as a separate process after the part is ejected from the mold or automatic de-gating may be accomplished with certain mold designs. For example, a tunnel-gate, also known as a “hook” or “submarine” gate, may be used to capture the runner in one part of the mold portion, so that when the molded part is ejected, the retained runner is pulled from the part. The tunnel-gate takes its name from the fact that the runner passage, which normally extends along the part line of the mold, “tunnels” below the part line into one mold part and then “hooks” upward into the mold cavity. The runner is removed from the mold with a slight twisting and/or flexing of the runner.
Automatic de-gating using, for example, a tunnel-gate, is often unsuccessful with elastic materials. Elastic materials, by stretching, dissipate the de-gating force developed as the part is ejected. The inability to de-gate such parts without a secondary operation, increases the cost of molding parts using elastic materials.
The present invention allows for automatic de-gating of parts molded of elastic materials. A runner support structure is molded within the runner passage from a material having low elasticity. The elastic material of the runner adheres to the runner support structure, the latter which serves to reinforce the runner. When the molded part is ejected, stretching of the runner is reduced, increasing the stress at the gate and ensuring proper de-gating. When the molded part is a “two shot” injection molding, using one shot of an elastic material and a second shot of a relatively inelastic material, the runner support structure may be molded during the injection of the inelastic material without the need to add process steps.
Specifically then, the present invention provides a method of de-gating a molded part produced by injection molding of an elastic material in a mold cavity. The method includes the steps of: (1) molding a runner support structure within a portion of the runner passage leading to the mold cavity by the injection of a substantially inelastic material into the runner passage, and (2) molding the part by the injection of elastic material into the mold cavity via the runner passage. The part is de-gated by a force on the runner formed in the runner passage, at least in part, transmitted through the runner support structure.
Thus, it is one object of the invention to provide automatic de-gating when molding parts from elastic materials. It is another object of the invention to allow runner passages to be freely designed without concern that cumulative stretch over the length of the runner may interfere with de-gating.
The method may be used in two shot molding operations employing an elastic and substantially inelastic material. In this case, the molding of the runner support structure may take place during the molding of the substantially inelastic material.
Thus, it is another object of the invention to provide a method of automatic de-gating of parts molded of elastic materials where the method is well adapted to two shot molding.
The step of forming the runner support structure may include fitting a blocking structure into a portion of the runner passage to prevent complete filling of the runner passage with the inelastic material during the molding of the runner support structure and then removing that blocking structure prior to molding with the elastic material. Conversely, the blocking structure may be fit into the portion of the runner passage during the molding with the elastic material and removed to allow the introduction of the inelastic material.
Thus, it is another object of the invention to make use of the same techniques used in two shot molding to produce the runner of the present invention.
The mold may have a part line separating two mold sections and the runner passage may be a tunnel-gate passing below the part line completely into one mold portion prior to entering the cavity.
Thus, it is another object of the invention to provide a de-gating system suitable for use with conventional tunnel-gate structures.
The runner support structure may be tapered in the direction of the gate.
Thus is another object of the invention to provide for a stress reducing interface between the relatively rigid runner support structure and the elastic portion of the runner such as minimizes separation or tearing at the interface between the runner support structure and the elastic material of the runner.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment and its particular objects and advantages do not define the scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.
Referring now to
After the runner 10 and tunnel-gate 18 are separated from the molded part, they may be removed from the mold section 16 by upward rotation of the runner 10 and/or flexing of the tunnel-gate 18 withdrawing it from its passage. To facilitate this process, the tunnel-gate 18 tapers to a narrower cross-section toward the molded part.
When a runner is composed of an elastic material, it may stretch sufficiently over its length to prevent the necessary concentration of stresses at the gate to allow automatic de-gating as described above. Accordingly, in the present invention, a multi-material runner is produced in which the elastic material of the runner is reinforced with a non-elastic material resisting runner stretching, better retention of the runner by the tunnel-gate, and improved ejection of the runner after molding.
In the following description and claims, the term “runner” will be used generally to refer both to runner 10 and also associated materials removed from the molded part including portions of the gate and/or sprue as context requires.
Referring now to
The elastic runner portion 22 includes a frusto-conical sprue 26 defined by a downwardly expanding passage receiving material directly from an injection nozzle (not shown). The sprue 26 connects to horizontally extending left and right runners 28 and 30 extending along a part line 12 of the mold (also not shown) and terminating in upwardly concave hook-shaped, left and right tunnel-gate portion 32 and 34, respectively. The outward tips of the left and right tunnel-gate portion 32 and 34 join to the molded part (not shown).
The under surface of the left and right runner 28 and 30 and the under surface of a portion of the left and right tunnel-gate portions 32 and 34 are joined to the upper surface of a conforming runner support structure 24. Specifically, the runner support structure 24 includes left and right extending runners 36 and 38, conforming and adhering, respectively, to the undersurface of upper, left and right runner 28 and 30. Likewise, left and right tunnel-gate portions 40 and 42 of the runner support structure 24 conform and adhere to undersurfaces of upper, left and right tunnel-gate portions 32 and 38.
As mentioned, the material of the runner support structure 24 is selected to be relatively inelastic so as to reduce the flexing and stretching of the elastic runner portion 22. The materials of the runner support structure 24 and elastic runner portion 22 are preferably selected to adhere to each other when molded together, however, materials having low adhesion may be accommodated by introducing a surface roughness or teeth, molded at the interface of these elements, to promote such adhesion.
Referring still to
The runner support structure 24 also includes a frusto-conical sprue 48, similar and parallel to sprue 26, but laterally displaced from the sprue 26 to receive a different injector nozzle (not shown). Sprue 48 joins to runners 36 and 38 by means of laterally extending offset portion 50.
Extending from the runners 36 and 38 in the opposite direction of offset portion 50 are auxiliary runners 52 and 54 which provide path to auxiliary gates (not shown) to a mold cavity as will be described.
Referring now to
When the invention is used in a two shot molding process, the runner support structure 24 may be molded at the time of the molding of the core elements, eliminating the need for additional process steps or materials. In this case, sprue 48 receives inelastic material into the offset portion 50 to form the lower left and right runners 36 and 38 and left and right tunnel-gate portions 40 and 42, and this same material is conducted through auxiliary runners 52 and 54 and auxiliary gates 67 to the part core 62.
Referring now to
Referring now to
A passageway 84 providing the remainder of the tunnel-gate is blocked from flow of material out of the runner support structure 24 by downwardly extending portions 86 of the second mold section 72. Similarly, flow of material from the core 62 into the passageway 84 is blocked by pins 88 of lateral mold sections 90 which press on the left and right about the first mold section 70.
Referring now to
The lateral mold sections 90 are replaced with mold sections 90′, removing blocking pins 88 and allowing flow of elastomeric material through the passageway 84 into an expanded mold cavity section 96 provided by the third mold section 92 and mold section 90′.
When the molded part 60 (including core 62 and elastomeric outer coating 66) is removed, the multi-material runner 20 (including runner support structure 24 and elastic runner portion 22) is retained by the first mold section 70, causing a de-gating at the points 98.
It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.